1. Field of the Invention
The present invention relates to a control device for a variable valve timing apparatus that changes valve timing of at least one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
Recently, more and more internal combustion engines of vehicles are provided with a variable valve timing apparatus that changes valve timing of an intake valve and an exhaust valve in order to increase the output, to improve fuel efficiency, or to reduce exhaust gas emission. Presently, actually used variable valve timing apparatuses are configured such that a rotational phase (camshaft phase) of a camshaft relative to a crankshaft is changed in order to change valve timing of the intake valve and the exhaust valve opened and closed by the camshaft. Actual valve timing (actual camshaft phase) is computed based on a crank angle signal outputted by a crank angle sensor and a cam angle signal outputted by a cam angle sensor during the operation of the internal combustion engine. Then, the variable valve timing apparatus is controlled such that the actual valve timing becomes target valve timing.
Also, JP-A-2001-82190 describes a technique, in which valve timing or the camshaft phase is forcibly changed to a limit position (for example, a full retard position) within a mechanically-movable range of the variable valve timing apparatus in order to correct computation error of the actual valve timing caused by manufacturing variations and aged deterioration of the variable valve timing apparatus. Then, a reference position for the valve timing is learned in the above forcibly changed situation, and then the actual valve timing is computed by referring to the learned reference position. For example, the actual valve timing on the basis of the learned reference position corresponds to an advance amount from the reference position (full retard position).
During starting of the internal combustion engine, the crank angle sensor outputs crank angle signals, and also the cam angle sensor outputs cam angle signals after the cranking of the engine has been started. However, before the cam angle sensor starts outputting the cam angle signals, the computation of the actual valve timing cannot be started disadvantageously because the actual valve timing is computed based on the output signals outputted from the crank angle sensor and the cam angle sensor.
As the countermeasure for the above disadvantages, the inventor of the present invention has been studying a system for obtaining the actual valve timing in an alternative method. More specifically, in the motor-actuated variable valve timing apparatus, during stopping of the internal combustion engine, a back-up RAM stores the actual valve timing that is computed based on the output signal outputted by the cam angle sensor immediately before the stop of the engine. Note that the back-up RAM is a memory capable of keeping the stored data by the assist of a vehicle battery as a power source for backing up the data even while the internal combustion engine is stopped. Then, during the starting of the internal combustion engine in a next operation, until a time at which the cam angle sensor starts outputting the cam angle signal, a valve timing change amount is computed based on a rotational speed of a motor of the variable valve timing apparatus, and the valve timing change amount is integrated to the value of the actual valve timing stored immediately before the stopping of the engine in the previous operation in order to obtain the actual valve timing.
However, even when the actual valve timing is once stored in the back-up RAM during stopping of the internal combustion engine, the stored data once stored in the back-up RAM may have been cleared due to the cut-off of the power source for backing up the data caused by detachment of the vehicle battery while the internal combustion engine is at rest. In the above case, when the internal combustion engine is to be started in the next operation, it is impossible to compute the actual valve timing disadvantageously until the cam angle sensor starts outputting the cam angle signals.
As the countermeasure for the above disadvantages, for example, a technique described in JP-A-2001-82190 may be employed. More specifically, during the starting of the internal combustion engine, the valve timing is forcibly changed to the limit position within the mechanically-movable range of the variable valve timing apparatus such that the valve timing is forcibly controlled to the reference position. Thus, detection of the present actual valve timing (reference position) is enabled before the cam angle sensor starts outputting the cam angle signals, and thereby the computation of the actual valve timing is started earlier.
However, recently, the movable range of valve timing is likely to be made larger in order to improve the fuel efficiency, and thereby the actual valve timing may be disadvantageously moved to certain valve timing that is beyond a range that enables the starting of the internal combustion engine. Thereby, if the actual valve timing is forcibly changed to the reference position at the start of the internal combustion engine, the startability of the engine may deteriorate disadvantageously.